Present day communication networks must generally serve end-users requiring a broad range of integrated (data/voice/video) services. For example, a large business with hundreds of employees and numerous host machines and communication needs for facsimile, high resolution graphics and video may require up to 1 gigabit/sec. transmission capability. This is in contrast to, for example, networks operating at speeds of 10 megabit/sec. for interconnecting terminals, intelligent workstations and personal computers to a large host or networks operating at speeds up to 50 megabit/sec. for interconnecting small numbers of large hosts
To achieve this gigabit/sec. capability, network processing previously performed electronically, such as transmitting, switching and receiving, is now implemented with optical processing, and the networks are configured with optical devices such as laser transmitters, photodiode receivers and fiber optic cables.
In many conventional electronic systems, a socalled orthogonal code was utilized for encoding incoming signals and a receiver was configured as a so-called correlation detector. The signals propagating over the channel facility could be expressed in terms of voltage and current waveforms. In the voltage-current domain, it is possible to have negative as well as positive voltages and currents. Because of these two polarities, the overlap or projection of one pattern from the orthogonal code onto a different pattern from the code over a given time interval (basically a correlation operation) could be reduced to essentially zero. This allowed for an effective detection process by the receiver since a high correlation implied the signal on the channel matched the receiver configuration whereas a low correlation indicated the signal was not destined for the particular receiver.
In an optical system, signals on a channel are propagated by the presence or absence of light energy or photons, that is, information interchange is conveyed by the ON/OFF signal states; this is in contrast to the plus, minus and zero states of electronic systems In optics, there is no equivalent to negative values. Thus, conventional electronic orthogonal code processing techniques cannot be exploited in the optical domain (or, for that matter, in any domain having a zero state and only one non-zero state).
In a multiple user environment having only two propagation states, signal separation and, ultimately, signal detection are generally achieved by time or frequency division, that is, a time or frequency slot is dedicated to each user. This is oftentimes inefficient. If a system has a large number of users, butonly a relatively small fraction of the users are active at one time, then the so-called code division scheme may be useful. Two versions of this technique are discussed in the article entitled "Coding and Decoding for Code Division Multiple User Communication Systems", published in the IEEE Transactions on Communications, vol. COM-33, No. 4, April, 1985. Decoding in a code division system is processing time is dedicated to decoding, thereby reducing